1. Field of the Invention
The present invention relates to an electrothermal printing apparatus using an ink ribbon composed of three layers; a resistive, a conductive and an ink, layer. In printing apparatus of this type, when local points on the resistive layer are heated by passing current through a number of printing pin electrodes arranged on an end edge of a printing head brought into contact with the resistive layer, ink applied on the ink layer is melted by heat and therefore transferred to recording paper to form an image, such as a character.
2. Description of the Prior Art
In the prior art electrothermal printing apparatus, a common return path forming element is usually provided. In more detail, in FIG. 1, an ink ribbon 4 is made up of a resistive layer 1, a conductive layer 2, and an ink layer 3. A drive circuit is made up of printing electrodes (e.g. 40 dots) 5 and a common return path forming element 6 both contactable with the resistive layer 1, and a plurality of transistors 7 each connected to each printing electrode 5.
A plurality of the printing electrodes 5 are embedded within an insulating material in such a way that the tip ends of the electrodes 5 are vertically spaced equal distance apart and exposed from an end edge surface of the printing head. The ink layer 3 is thermally transferable ink material (e.g. heat fusable ink). The conductive layer 2 is aluminum. Each of the printing electrodes 5 is connected to each collector of each transistor 7, while the common return element 6 is grounded. A supply voltage (+V) is supplied to each emitter of each transistor 7, and a switching control circuit (not shown) is connected to bases of these transistors 7.
In operation, these transistors 7 are selectively turned on or off in response to control signals from the switching control circuit, for providing printing and non-printing operations. Upon turning-on of the transistor 7 and consequent energization of the corresponding printing electrode 5, current will flow from the electrode 5 via the resistive layer 1 and conductive layer 2 to the common return path forming element 6. When current flows through the resistive layer 1, power dissipated will cause heating of the resistive layer 1 extending from the tip end of the printing electrode 5 to the adjacent portion of the conductive layer 2. This localized heating of the resistive layer 1 by the electrical power dissipation will case melting of the thermally transferable material on the ink layer 3 and thereby form an image on a recording paper 8.
By concurrent energization of selected printing electrodes 5 during movement of the print head in a direction relative to the ink ribbon 4 and recording paper 8, a desired image (e.g. character) can be imprinted on the recording paper 8.
In the prior-art printing apparatus as described above, however, there exist a few drawbacks as follows:
(1) The common return path forming element 6 is readily heated by the common return current, thus increasing power loss therethrough and therefore deteriorating printing efficiency.
(2) The arrangement of the common return path forming element 6 in addition to the printing electrodes 5 inevitably increases the number of necessary parts, thus increasing the manufacturing cost thereof and decreasing the reliability thereof, because the element 6 is in slidable contact with the resistive layer 1.
(3When a great number of printing images are required to form by combining these printing electrodes 5, the presence of the common return path forming element 6 causes some restriction from design point of view.
The more detailed description of the above-mentioned prior-art printing apparatus is made in U.S. Pat. No. 4,350,449 by Countryman et al, thereby incorporated by reference herein.